After a recent hard rain, I noticed that the floor in the cargo area of our 2014 Outback was wet. Inspection of the hatch seal did not show any damage so I decided the most likely location for the leak was from around the 3rd brake light. The procedure below describes what I did to fix the leak but the same steps would be used to replace the brake light assembly.
Step 1: Removing Upper Trim
To access the brake light, you need to remove the upper trim piece around the rear window. The trim is held in place with 5 or 6 clips. To remove the trim, gently pry the trim up around the perimeter to release the clips. I used some plastic interior trim removal tools. They are fairly cheap to buy at your local auto parts store and won't scratch the other trim and glass. You could use a screwdriver or scraper but use caution to avoid damage.
Once you release one clip, work your way around the trim until all the clips are loose. The trim can then be removed. You can see in the last picture that the inside of the trim had quite a bit of water in it.
Step 2: Removing the Brake Light and Repairing the Leak
To remove the brake light, you need to take the (2) 8mm retaining nuts off. There are 2 circular holes in the hatch at either end of the light that allow access to the nuts. Once the nuts have been removed, the light pulls out from the outside of the hatch. To remove the light completely, push the locking clip in on the light connector and remove the plug.
I found that one of the nuts on the light was very loose and most likely the reason it was leaking. I decided to pull the light out and applied a very small bead of clear silicone around the openings for the light and mounting studs to ensure a good seal.
One thing I noticed is that there is no way to replace the bulbs in the upper brake light and you have to replace the entire light assembly if the lights are inoperative. Being they are LED's, hopefully that doesn't happen often.
Step 3: Reinstallation
The wiring harness for the brake light was reconnected and the light assembly was reinstalled. Make sure it is centered in the opening. Reinstall the retaining nuts being careful to not over tighten them.
To reinstall the trim, align the trim in place and push the clips into the receptacles. It should click into place without much effort. Test the brake light and that's it.
Now I just need to wait for the next rain storm!